The primary research objective is to estimate the relative contributions of genetics and environment to the determination of blood pressure levels among a group of newborn twins. Blood pressure is measured on days 0, 1, 2, 3, 14 and 1, 3, 6, 9 and 12 months. Zygosity is determined through blood group analysis, dermatoglyphics, physical appearance, and placental assessment. Placentation is also analyzed for each twin pair with respect to intravascular communications. Additionally, parents and siblings of the twins are evaluated as to blood pressure, height and weight. The ultimate purpose of this research is to shed light on the etiology of blood pressure levels in order to aid in the identification of individuals most likely to experience elevated blood pressure.